According to Levinson (1983, 2004) Discourse Deixis refers to the use of expressions that signal a relationship between an utterance and the prior or subsequent discourse.
Discourse deixis includes temporal deictic terms (next, before, last week), spatial deictic terms (in the last paragraph), utterance initial expressions that indicate a relationship with the preceding discourse (but, however, in conclusion, well), and the use of demonstratives (this, that) to refer to the preceding or following discourse (Levinson 1983, 2004).
And by the way, we will do that.
The word "That" in the statement above refers to utterances, "And i will say this, Secretary Kelly will be the man to do it, and we will give him a wall, and it will be a great wall". The word "That" here states by Donald Trump to explain that he will give Secretary a wall as a separator between America and Mexico. Because the word "That" refers to the statement above, it could be categories as discourse deixis.
And this is for the security of the country.
The word "This" in the statement above refers to "He may by proclamation and for such period as he shall deem necessary --- so here it is, people coming in --- suspend the entry of all aliens Right? That's what it says. It's not like --- again, a bad high school student would understand this. Anybody would understand this.
Suspend the entry of all aliens or any class of aliens as immigrants or nonimmigrants, or impose on the entry of aliens. Okay, so you can suspend the aliens, right? You can suspend the aliens from coming in --- very strong --- or impose on the entry of aliens any restrictions he may deem to be appropriate.
Okay. So you can suspend, you can put restrictions, you can do whatever you want. And this is for the security of the country". In this statement, the speaker states that he and his government and also the people from police association can suspend and put restrictions for security of the country.